The Enigmatic
Diffuse Interstellar Bands:
A Reservoir of Organic Material
Ben McCall
Department of Chemistry and Department of Astronomy
University of Illinois at Urbana-Champaign
Apache Point Observatory DIB Collaboration:
Meredith Drosback (Colorado), Scott Friedman (STScI), Lew Hobbs (Yerkes), Ben McCall
(UIUC), Takeshi Oka (Chicago), Brian Rachford (ERAU), Ted Snow (Colorado), Paule
Sonnentrucker (JHU), Julie Thorburn (Yerkes), Dan Welty (Chicago), Don York (Chicago)
pre
Astro biology
^
• Precursors in diffuse
clouds?
• Precursors in dense
clouds?
• Precursors in disks?
• Delivery to planets?
• Production of pre-biotic
molecules on planets
• Formation of life
Dense vs. Diffuse Clouds
Dense molecular clouds:
•
•
•
•
H  H2
C  CO
n(H2) ~ 104–106 cm-3
T ~ 20 K
Diffuse clouds:
•
•
•
•
H ↔ H2
C  C+
n(H2) ~ 101–103 cm-3
T ~ 50 K
 Persei
Pound
ApJ 493, L113 (1998)
Photo: Jose Fernandez Garcia
Molecules in Diffuse Clouds
H· 3×109 M = 6×1042 g ~ 3×1066 atoms
Optical:
OH
CH+
C2
CN
CH
C3
NH
~ 1×1059
~ 8×1058
~ 5×1058
~ 7×1057
~ 5×1057
~ 4×1057
~ 2×1057
DIBs ~ 1058 ?
Ultraviolet:
H2
CO
HD
N2
HCl
~ 1×1066
~ 2×1061
~ 1×1060
~ 9×1058
~ 6×1056
Infrared:
H3+
~ 2×1059
mm-wave:
C2H
H2CO
HCN
CS
HCO+
C3H2
HNC
HOC+
~ 3×1058
~ 1×1058
~ 8×1057
~ 5×1057
~ 5×1057
~ 2×1057
~ 1×1057
~ 8×1055
Based on abundances from Snow & McCall, ARAA 44, 367 (2006)
Discovery of the DIBs
• 5780, 5797 seen as unidentified bands
–  Per,  Leo (Mary Lea Heger, Lick, 1919)
• Broad (“diffuse”)
• Possibly “stationary”
B. J. McCall, in preparation
Interstellar Origin
P. W. Merrill, ApJ 83, 126 (1936)
P. W. Merrill, PASP 483, 179 (1936)
DIBs as Radicals or Ions?
100
50
200
150
1920
1940
1960
1980
350
300
250
0
2000
Hobbs et al. 2008
Tuairisg et al. 2000
Jenniskens & Desert 1994
Herbig 1988
Herbig 1975
Merrill & Wilson 1960
Herbig 1966
Merrill & Wilson 1938
Heger 1919
# of known DIBs
A Growing Problem
• Mostly in the visible
– 4175–8763 Å
• None known in UV
• Few in near-infrared
– 9577, 9632, 11797, 13175 Å
Examples: Lorentzian Profiles
0.38 ps
Snow, Zukowski, & Massey, ApJ 578, 877 (2002)
Examples: Smooth Profiles
M. Drosback, Ph.D. Thesis (2006)
Krelowski & Schmidt, ApJ 477, 209 (1997)
Examples: Fine Structure
Galazutdinov et al., MNRAS 345, 365 (2003)
Examples: Molecular Structure?
Sarre et al., MNRAS 277, L41 (1995)
Galazutdinov et al., MNRAS 345, 365 (2003)
Kerr et al., MNRAS 283, L105 (1996)
The APO DIB Survey
•
•
•
•
•
Apache Point Observatory 3.5-meter
3,600–10,200 Å ; / ~ 37,500 (8 km/s)
119 nights, from Jan 1999 to Jan 2003
S/N (@ 5780Å) > 500 for 160 stars (115 reddened)
Measurements & analysis still underway
Echelle Image
Fraunhofer Lines
H K
H: Ca II ~ 3968 Å
K: Ca II ~ 3934 Å
D
D: Na I ~ 5890 Å
A
A: O2 ~ 7650 Å
Stellar Spectra
C
60000
HD 50064
EB-V = 0.84
50000
30000
F
20000
B
10000
0
KH
4000
D
6000
A
8000
Wavelength (Å)
10000
120000
100000
Intensity
Intensity
40000
Cyg OB2 #12
EB-V = 3.2
80000
60000
40000
20000
0
4000
6000
8000
Wavelength (Å)
10000
Stellar Spectra
 Orionis
HD 183143
1.0
4428
0.6
5780, 5797
1.0
0.4
Relative Intensity
1.00
Relative Intensity
Relative Intensity
0.8
0.95
CH
0.90
0.9
0.8
0.7
0.2
0.85
4299
4000
4300
4301
Wavelength (Å)
4500
4302
5760
5000
Wavelength (Å)
5770
5780
5790
Wavelength (Å)
5500
5800
6000
Overview of Preliminary Results
• Spectral atlas of DIBs
– for comparison to laboratory spectra
– Version 1: HD 204827
– Version 2: Four reddened stars
• DIB correlations
– between DIBs & other species
→ chemistry, environment of carriers
– among DIBs
→ spectra of carriers
DIB Spectral Atlas: Version 1
• Initial target: HD 204827
–
–
–
–
heavily reddened: EB-V=1.11
early spectral type: ~B0V
fairly bright: V=7.94
member of open cluster Trumpler 37
• in Cepheus OB2 association
• Abundant C2
• Champion of C3
Adamkovics, Blake, & McCall, ApJ 595, 235 (2003)
Spectroscopic Binary!
10 Lac
avg
Night 5
Night 4
Night 3
Night 2
Night 1
Great Test for DIBs
• DIB count: 267 confirmed, 113 new, 380 total!
• ApJ in press (see http://dibdata.org)
DIB Spectral Atlas: Version 2
Four heavily reddened stars:
Star
Sp. Type
EB-V
N(C2)
HD 204827
B0V
1.11
440×1012
Cyg OB2 5
O7f
1.99
200×1012
HD 166734
O8e
1.39
160×1012
HD 183143
B7Iae
1.27
< 6×1012
Sample of Spectra
reddened
previous
work
DIBs
unreddened
Statistics & Status
Criterion
(2 stars)
Criterion
(4 stars)
New Confirmed Total
DIBs
DIBs
DIBs
10 σ
5σ
111
270
381
8σ
4σ
151
291
442
4σ
―
284
350
634
• Issues still to address:
– defining the continuum
– blends of DIBs with each other
• Complete atlas ~late 2008?
DIB Correlations: H & H2
λ5780 well correlated with H
no correlation with H2
[a la Herbig ApJ 407, 142 (1993)]
r=0.944
(linear fit,
w/o outliers)
York et al., in preparation
r=0.589
(linear fit)
The “C2 DIBs”
• First set of DIBs known to be correlated with a
known species!
Tuairisg atlas
1.5
1.4
Relative Intensity
N(C 2) (10
1.3
12
HD 183143
<3
HD 167971
<4
HD 179406
73
HD 206267
93
HD 34078
-2
EB-V
cm )
1.27
1.08
0.33
0.53
110
0.52
1.2
HD 147889
210
HD 172028
1.1
1.07
270
0.79
HD 204827
1.0
430
1.11
0.9
4960
4965
4970
4975
4980
4985
5170
Wavelength (Å)
5180
5540
5550
Thorburn et al, ApJ 584, 339 (2003)
Correlations Among DIBs
• Assumptions:
– gas phase molecules
– DIBs are vibronic bands
– low temperature
• carriers all in v=0
– relative intensities fixed
A
v=0
• Franck-Condon factors
• independent of T, n
• Method:
– look for DIBs with tight
correlations in intensity
• Prospect:
– identify vibronic spectrum of
single carrier
– spacings may suggest ID
X
v=0
A
Star #1
1
2
3
Intensity
Star #2
B
Strength of DIB B
Correlation Plots
Star #3
Wavelength
Strength of DIB A
Example: Bad Correlation
r=0.55
Example: Good Correlation
r=0.985
measurement errors
could be causing deviations?
Statistics of Correlations
• 58 strong DIBs
• Pairs of DIBs
observed in >40 stars
• 1218 pairs
• Generally well
correlated
• Few very good
correlations
– 118 with r > 0.90
– 19 with r > 0.95
Why So Few Perfect Correlations?
• Assumption of a common ground state bad?
– energetically accessible excited states?
• spin-orbit splitting?
– if linear molecules
• low lying vibrationally excited states?
– if very large molecules
• “Vertical” transitions?
– intense origin band
– weaker vibronic bands
• correlations could be seen with weaker bands?
• Lots of individual molecular carriers?
The Road to a Solution
• Laboratory spectroscopy is essential!
• Blind laboratory searches unlikely to work
~107 organic molecules known on Earth
~10200 stable molecules of weight < 750
containing only C, H, N, O, S
• Observational constraints & progress are
also essential!
• Computational chemistry will play an
important role
• Close collaborations needed!
• Great astrobiological significance!
Acknowledgments
ACS Petroleum
Research Fund
Dreyfus New
Faculty Award
NASA Laboratory
Astrophysics
NSF Divisions of
Chemistry & Astronomy
Packard
Fellowship
Air Force Young
Investigator Award
Cottrell
Scholarship
http://astrochemistry.uiuc.edu
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SCRIBES: Sensitive Cooled Resolved Ion BEam Spectroscopy
• Infrared spectra of ions important in:
– astrochemistry
– atmospheric chemistry
– propulsion/combustion
• Optical spectra → DIBs ?
Evaluation of Proposed DIB Carriers
• Need a laboratory spectrum
HD 185418
– gas phase (avoid matrix shifts)
– rotationally resolved (or profile resolved)
HD 229059
1.0
– interstellar temperatures, excitation conditions
• DIB, simulated spectra must match exactly
– central wavelength & profile
– relative intensities & correlation
– all laboratory bands present
Relative Intensity
• Need to be able to simulate spectrum
70 K
0.9
50 K
30 K
0.8
10 K
C76268
6270
6272
Wavelength (Å)
McCall et al., ApJ 559, L49 (2001)
l-C3H2-
McCall et al., ApJ 567, L145 (2002)
Carbon Chains as DIB Carriers?
• Some DIBs correlated with C2
• C3 widely observed in diffuse clouds
• But, search for C4, C5
unsuccessful so far
• Conclusions:
– Need high abundance, or
– Large oscillator strength
– Maier, Walker, & Bohlender [ApJ 602, 286 (2004)]:
• Potential carbon chain DIB carriers must have >15 carbon atoms
• C2n+1 (n=7-15); HCnH (n>40); C2n (n>10); CnH; HCnH+; Cn-
– No lab spectra of long chains; very little of cations
Ádaámkovics, Blake, &McCall,
ApJ 595, 235 (2003)
– J. P. Maier 2001
PAHs as DIB Carriers?
• Polycyclic Aromatic Hydrocarbons
– proposed by Leger & d’Hendecourt and by
van der Zwet & Allamandola in 1985
• Would expect complex mixture
– ionization stages (cation, neutral, anion?)
– hydrogenation states
• So far, no spectroscopic match with DIBs
• Cation transitions observed so far in gasphase are too broad!
• Still no convincing evidence
Fullerenes as DIB Carriers?
• Expected to form in carbon star outflows
• IP(C60) = 7.6 eV
– Ionized in diffuse clouds
Fulara, Jakobi, & Maier
• C60+ in Ne matrix
Chem. Phys. Lett. 211, 227 (1993)
– two bands near 9600 Å
Foing & Ehrenfreund
• Detection claimed in HD 183143 A&A
319, L59 (1997)
• Need gas-phase spectrum!
– Experiment in
preparation
HD 183143
C60+
Ne matrix
C2 DIBs toward HD 62542
• Unusual sightline with only diffuse cloud “core”
– outer layers stripped away by shock (?)
– DIBs undetected [Snow et al. ApJ 573, 670 (2002)]
• Recent Keck observations (higher S/N)
– Classical DIBs (e.g. λ5780) very weak
– C2 DIBs among the few DIBs observed
• C2 DIBs evidently form in denser (molecular) regions
Ádámkovics, Blake, & McCall ApJ 625, 857 (2005)
DIB "Families"
• Look for sets of DIBs in which all correlation
coefficients are high
λ6196
λ4963
λ6613
0.96
0.92
λ5487
0.96
λ6204
λ4727
blend
λ5780
0.97
0.94
0.94
0.93
λ4984
0.86
0.92
λ5418
blend
λ6284
More on λ6613 & λ6196
Can observed scatter be due to measurement errors?
•
•
•
•
•
•
•
Observed r=0.985
Assume perfection
Add Gaussian noise
1000 M.C. trials
Double the noise
1000 M.C. trials
Statistically OK if we
underestimated errors
r=0.985
r=0.996
Measurement Errors
• Is doubling the error estimate reasonable?
• Error sources include:
– finite S/N
– unexpected structure
• Agreement not
always perfect!
– interfering DIBs?
– continuum placement
HD 281159
– CH+ A-X band
• Hard to say; but
it’s certainly a
very good
correlation!
r=0.985